Akai Houseki No Himitsu summary:

Susannah is an editor. During a party at a book fair, she sees a portrait of a woman who is wearing the same red necklace as her. Intrigued, Susannah steps closer to the painting, only to be engulfed by a mysterious light that transports her 100 years back in time! To make matters worse, Kane Wilder, CEO of an IT company, who is convinced Susannah is his brother's mistress, accidentally travels with her!